1. Field of the Invention
The present invention relates to liquid crystal display (LCD) devices. More particularly, the invention relates to an active-matrix-type LCD device which supplies signal potentials to signal lines of an LCD panel according to a time-division drive method.
2. Description of the Related Art
Currently, active-matrix-type LCD devices are dominantly used as LCD devices for use in personal computers and word processors. The active-matrix-type LCD devices exhibit excellent response speed and image quality characteristics, and are thus suitable for use in color-type LCD devices which have recently been put into practical use. In this type of device, non-linear devices, such as transistors or diodes, are used for the individual pixels of an LCD panel, and more specifically, thin film transistors (TFTs) are formed on a transparent insulating substrate (for example, a glass substrate).
In LCD devices, and in particular, in large LCD devices, a driver IC, which is a horizontal drive circuit for sequentially supplying signal potentials to lines of pixels, is formed on an external circuit board, which is provided separately from the transparent insulating substrate on which the LCD panel is formed. Generally, outputs of the external driver IC and signal lines of the LCD panel have a one-to-one relationship. That is, a signal potential output from each output terminal of the driver IC is supplied to the corresponding signal line.
In contrast, with a view to achieving the miniaturization of the driver IC, a time-division drive method is known for driving an LCD panel, which allows the number of output pins i.e., (output terminals) of the driver IC to be reduced. In this method, a plurality of signal lines are collected as one unit block, and a signal potential to be supplied to one block of the signal lines is output from the driver IC in time series. Meanwhile, a time-division switch is provided for the LCD panel so as to time-divide the time-series signal potentials output from the driver IC, thereby sequentially supplying the divided signal potentials to the corresponding signal lines.
The following drive methods may be employed in the above-described type of LCD device using the time-division drive method. In one method, the polarity of image data to be supplied to each pixel is inverted in every horizontal scanning (1H) period for a common voltage VCOM, which is referred to as the xe2x80x9c1H inversion drive methodxe2x80x9d. In another method, the common voltage VCOM is AC-inverted in every 1H period, which is referred to as the xe2x80x9c1H common (VCOM) inversion drive methodxe2x80x9d. If the 1H inversion drive method is used singly or in combination with the 1H common inversion drive method for the above type of liquid crystal device, fluctuations of the writing potential caused by crosstalk of the signal potential from a selected signal line to a non-selected signal line cannot be ignored. The reason for this is discussed below in detail with reference to FIG. 12 illustrating the configuration of the time-division switch.
In FIG. 12, a time-division switch 101 is formed of a CMOS analog switch formed by connecting an NchMOS transistor and a PchMOS transistor in parallel to each other. The time-division switch 101 is connected between a common signal line 102 for transmitting a signal voltage output from a driver IC (not shown) and a signal line 103 of an LCD panel. With this arrangement, by applying a select pulse S and its inverted pulse XS to the gates of the respective NchMOS transistor and PchMOS transistor, the time-division switch 101 transmits a signal voltage from the driver IC to the signal line 103.
As discussed above, due to the crosstalk of the signal potential from a selected signal line to a non-selected signal line, the writing potential is changed. Then, the signal potential of the non-selected signal line becomes lower, as illustrated in FIG. 13, with respect to a ground potential (0 V). Then, the gate potential of the NchMOS transistor becomes positive with respect to the potential of the signal line, i.e., to the source potential of the NchMOS transistor. This potential relationship satisfies the condition of switching on (conducting) the NchMOS transistor. As a result, the NchMOS transistor is activated.
This causes the signal charge to flow out of the non-selected signal line via the NchMOS transistor, thereby lowering the signal potential of the non-selected signal line. As a consequence, a resulting image suffers from insufficient contrast and non-uniformity of the luminance in the horizontal direction, thereby degrading the image quality.
Accordingly, in view of the above background, it is an object of the present invention to provide an LCD device that maintains a high image quality by eliminating the generation of insufficient contrast and non-uniformity of the luminance in the horizontal direction caused by the crosstalk of a signal potential from a selected signal line to a non-selected signal line.
In order to achieve the above object, according to the present invention, there is provided a liquid crystal display device including a first substrate having a display unit on which pixels are formed at intersections of gate lines for a plurality of rows and signal lines for a plurality of columns, the gate lines and the signal lines being arranged in a matrix. A vertical drive circuit is mounted on the first substrate so as to drive the gate lines. A horizontal drive circuit outputs a time-series signal potential in correspondence with a predetermined number of time-divided portions. A time-division switch time-divides the time-series signal potential output from the horizontal drive circuit and supplies the divided time-series signal potential to a given signal line among the signal lines. A select pulse generating circuit generates a select pulse for activating the time-division switch. A low-level potential of the select pulse is set to be lower than a low-level potential of the signal potential output from the horizontal drive circuit. A second substrate opposes the first substrate with a predetermined gap therebetween. A liquid crystal layer is encapsulated between the first substrate and the second substrate.
According to the above-described liquid crystal display device, if a signal potential is transferred from a selected signal line to a non-selected signal line, the potential of the non-selected signal line is reduced. This further decreases the source potential of an NchMOS transistor of a CMOS transistor, which is used as the time-division switch. In the above arrangement, however, the low-level potential of the select pulses to be applied to the gate of the NchMOS transistor is lower than the low-level potential of the signal potential. Accordingly, the source potential of the NchMOS transistor does not become lower than the gate potential, which would otherwise activate the NchMOS transistor and cause the charge to flow out of the non-selected signal line via the NchMOS transistor. As a consequence, the potential of the non-selected signal line can be maintained at the initially written potential.